In recent years, in the information recording field, a variety of attempts of research and development have be made for the information recording system. In the optical information recording system, data can be recorded and reproduced contact-freely. As one advantage, the optical information recording system can accomplish a recording density more than ten times higher than the conventional magnetic recording system. As another advantage, the optical information recording system can apply to individual memory types such as the read-only type, the write-once type, the rewritable type, and so forth. Thus, as a system that accomplishes inexpensive and large-capacity files, it has been considered that the optical information recording system will be widely used from industries to homes.
Among those, digital audio discs (DAD) and optical video discs, which are optical discs of read-only memory type, have become common.
In an optical disc such as a digital audio disc, a reflection film that is a metal thin film such as an aluminum (Al) film is formed on an optical disc substrate that is a transparent substrate on which an uneven pattern such as pits and grooves that represent an information signal is formed. In addition, a protection film that protects the reflection film against moisture (H2O) and oxygen (O2) in the atmosphere is formed on the reflection film. When an information signal is reproduced from the optical disc, read light such as laser light is radiated from the optical disc substrate side to the uneven pattern. Corresponding to the difference of reflectance of the incident light and the reflected light of the read light, the information signal is detected.
When such an optical disc is produced, an optical disc substrate having an uneven pattern is formed by the injection molding method. Thereafter, a reflection film made of a metal thin film is formed on the optical disc substrate by the vacuum evaporation method. Thereafter, an ultraviolet ray setting resin is coated on the reflection film. As a result, the protection film has been formed.
In the aforementioned optical information recording system, in recent years, an increase of recording density has been required. To satisfy that, a technology for decreasing the diameter of the spot of the read light with an increased numerical aperture (NA) of an objective lens through which the read light is radiated from an optical pickup has been proposed. According to the technology, while the NA of an objective lens used to reproduce data from a conventional digital audio disc is 0.45, the NA of an objective lens used to reproduce data from an optical video disc such as a DVD (Digital Versatile Disc) that has a recording capacity six to eight times higher than that of the digital audio disc is around 0.60. As a result, the diameter of the spot is decreased.
As the NA of the objective lens is increased, the disc substrate of the optical disc should be thinned so as to transmit the radiated read light. This is because the tolerable amount of the deviation angle (tilt angle) of the optical axis of the optical pickup against the vertical line of the disc surface becomes small. The tilt angle is subject to influences of aberration and double refraction due to the thickness of the substrate. Thus, the substrate is thinned so that the tilt angle becomes as small as possible. For example, the thickness of the substrate of the aforementioned digital audio disc is around 1.2 mm. In contrast, the thickness of the substrate of an optical video disc such as a DVD, which has a recording capacity six to eight times higher than the digital audio disc, is around 0.6 mm.
However, in consideration of the increasing needs of high recording density, the substrate should be further thinned. To satisfy that, another type optical disc has been proposed. In this optical disc, an uneven pattern is formed on one main plane of a substrate as an information signal portion. A reflection film and a light transmission layer, which is a thin film that transmits light, are successively layered. Read light is radiated from the light transmission layer side. As a result, an information signal is reproduced. In such an optical disc, of which read light is radiated from the light transmission layer side and thereby an information signal is reproduced, the film thickness of the light transmission layer is decreased so that the optical disc can conform with a large NA of an objective lens.
However, when the light transmission layer is thinned, it becomes difficult to form a light transmission layer with a thermoplastic resin by the injection molding method, which is used in a conventional optical disc producing method. In other words, in the conventional technology, it is very difficult to form a light transmission layer that has a thickness of around 0.1 mm and that has small double refraction and good transparency.
To solve such a problem, a method for forming a light transmission layer with an ultraviolet ray setting resin. In this method, however, when the light transmission layer is formed with the ultraviolet ray setting resin, it is very difficult to keep the film thickness of the light transmission layer constant on the surface of the substrate. Thus, it is difficult to stably reproduce the information signal.
Alternatively, the light transmission layer may be formed by adhering a thermoplastic resin sheet having a film thickness of 0.1 mm to the surface of the substrate by the roller pressing method. However, in this method, when the sheet is pressed, the sheet deforms and the adhesive agent protrudes to the reading surface side. Thus, it is difficult to form the light transmission layer with an equal film thickness. It is much difficult to stably reproduce the information signal.
Thus, to solve those problems, the inventors of the present invention have thought of a method for forming a light transmission layer of which a sheet composed of an adhesive layer and a light transmission layer is adhered to an information signal portion of a substrate. As a result, a method for adhering a light transmission layer to a substrate using an adhering device composed of an elastic pad and a metal plane stage has been proposed. Next, with reference to an accompanying drawing, the adhering device will be practically described.
In other words, as shown in FIG. 1, in a conventional adhering device, a fixed stage 101 and a movable stage 102 are disposed at their opposite positions.
The fixed stage 101 is used to hold a sheet 103. Thus, the fixed stage 101 is structured so that it can hold the sheet 103. In other words, a vertically movable pin 105 that is protruded from the fixed stage 101 and buried therein is disposed in the fixed stage 101. The vertically movable pin 105 faces the movable stage 102. The diameter of the vertically movable pin 105 is the same as the diameter of a through-hole 103a of the sheet 103. The through-hole 103a of the sheet 103 is fit to the vertically movable pin 105. As a result, the sheet 103 can be held on the fixed stage 101. A substrate alignment pin 106 cylindrically protrudes at an upper portion of the vertically movable pin 105. The diameter of the substrate alignment pin 106 is almost the same as the diameter of a center hole 104a of a disc substrate 104. While the substrate alignment pin 106 aligns the center of the disc substrate 104, the vertically movable pin 105 holds the disc substrate 104. On the fixed stage 101, the sheet 103 is fit to the substrate alignment pin 106 so that the sheet 103 is hold by a shoulder portion of the vertically movable pin 105.
A pad 107 made of an elastic member such as rubber is disposed on an inner plane of the movable stage 102 so that the pad 107 faces the fixed stage 101. The pad 107 is formed in a conical shape. The plane side of the conical shape is secured to the inner plane of the movable stage 102.
When the disc substrate 104 and the sheet 103 are adhered by such an adhering device, the through-hole 103a of the sheet 103 is fit to the vertically movable pin 105 disposed so that the sheet 103 is held on the fixed stage 101. At that point, an adhesive plane 103b of the sheet 103 is disposed so that it faces the movable stage 102. Thereafter, the center hole 104a of the disc substrate 104 is fit to the substrate alignment pin 106 so that the center hole 104a is held on the shoulder portion of the vertically movable pin 105. At that point, the disc substrate 104 is held on the vertically movable pin 105 so that a recording plane 104b that has an information signal portion faces the adhesive plane 103b that has an adhesive layer.
Thereafter, the movable stage 102 is moved toward the fixed stage 101 (downwards in FIG. 1). With the pad 107, the substrate alignment pin 106 is pressed. Thereafter, the vertically movable pin 105 is moved in the fixed stage 101 through the disc substrate 104. As a result, the clearance between the disc substrate 104 and the sheet 103 gradually becomes small. Finally, the disc substrate 104 and the sheet 103 are press-fit. As a result, the recording plane 104b and the adhesive plane 103b are adhered. After they have been stably press-fit, the movable stage 102 is moved in the direction that it is apart from the fixed stage 101. Thereafter, with a predetermined conveying device (not shown), the disc substrate 104 and the sheet 103, which have been press-fit, are removed from the fixed stage 101.
As a result, the disc substrate 104 and the sheet 103 have been adhered. An optical disc of which a light transmission layer has been formed on the recording plane 104b of the disc substrate 104 has been produced.
An optical disc having a light transmission layer formed in such a manner can conform with a large NA of the objective lens, which is used to reproduce data from the optical disc.
However, various experiments and evaluations that the inventors have repeatedly conducted show that such an optical disc has the following problem.
In other words, when a phase change type recording material is used for an information signal portion in an optical disc on which a light transmission layer is formed, as the material of the outermost layer of the information signal portion, a mixture (ZnS—SiO2) of zinc sulfide and silicon oxide, which is a transparent dielectric, are normally used. A light transmissivity sheet is adhered onto the information signal portion having the ZnS—SiO2 layer as the outermost layer through an adhesive layer made of a pressure-sensitive adhesive agent is disposed.
However, the results of which the inventors of the present invention have conducted acceleration tests for a plurality of optical discs having such a light transmission layer and measured reflectance thereof show that the reflectance thereof deteriorates. When the reflectance deteriorates, it will become difficult to record and reproduce an information signal to and from an optical disc having a light transmission layer with high accuracy.
Therefore, an object of the present invention is to provide an optical disc of which a light transmissivity sheet has been adhered on one main plane of a substrate through an adhesive layer, in particular, an optical disc that suppresses the variation of reflectance thereof, that suppresses the variation of reflectance on the recording/reproducing plane thereof, that conforms with a large NA of an objective lens used to record/reproduce data, and that has a light transmission layer that has small double refraction, high transparency, and equal film thickness and an optical disc producing method that allows the production yield to improve.